The invention is directed to a single mode optical waveguide fiber, more particularly to a waveguide fiber in which the total dispersion is maintained positive over the entire fiber length. In addition, the effective area is high and total dispersion slope is maintained at a low value.
Because of the high data rates and the need for long regenerator spacing, the search for high performance optical waveguide fibers designed for long distance, high bit rate telecommunications has intensified. An additional requirement is that the waveguide fiber be compatible with optical amplifiers, which typically show an optimum gain curve in the wavelength range 1530 nm to 1570 nm. Consideration is also given to the potential of expanding the usable wavelength into the L-Band range of about 1570 nm to 1700 nm, more preferably in the range of about 1570 nm to 1625 nm.
In cases where waveguide information capacity is increased by means of wavelength division multiplexing (WDM) technology, an additional waveguide fiber property becomes important. For WDM, high bit rate systems, the waveguide should have exceptionally low, but non-zero, total dispersion, thereby limiting the non-linear dispersion effect of four wave mixing.
Another non-linear effect which can produce unacceptable dispersion in systems having a high power density, i.e., a high power per unit area, is self phase modulation. Self phase modulation may be controlled by designing a waveguide core which has a large effective area, thereby reducing the power density. An alternative approach is to control the sign of the total dispersion of the waveguide so that the total dispersion of the waveguide serves to counteract the dispersive effect of self phase modulation.
A waveguide having a positive dispersion, where positive means shorter wavelength signals travel at higher speed than those of longer wavelength, will produce a dispersion effect opposite that of self phase modulation, thereby substantially eliminating self phase modulation dispersion.
Such a waveguide fiber is disclosed and described in U.S. patent application Ser. No. 08/559,954, now U.S. Pat. No. 5,748,824. The present novel profile improves upon the 08/559,954 now U.S. Pat. No. 5,748,824 fiber by increasing effective area. In addition the waveguide of this disclosure has a total dispersion over the wavelength window of operation that is everywhere positive and has a lower limit greater than about 2.0 ps/nm-km to further reduce the power penalty due to four wave mixing.
Thus there is a need for an optical waveguide fiber which:
is single mode over at least the wavelength range 1530 nm to 1570 nm; PA1 has a zero dispersion wavelength outside the range 1530 nm to 1570 nm; PA1 has a positive total dispersion over the wavelength range 1530 nm to 1570 nm which is not less than about 2.0 ps/nm-km but yet is low enough to avoid a large linear dispersion power penalty; PA1 has a usable transmission window in the range of about 1570 nm to 1625 nm; and PA1 retains the usual high performance waveguide characteristics such as high strength, low attenuation and acceptable resistance to bend induced loss. PA1 The refractive index profile is the relationship between refractive index and waveguide fiber radius. PA1 A segmented core is one that has at least a first and a second waveguide fiber core radius segment. Each radius segment has a respective refractive index profile. PA1 The radii of the segments of the core are defined in terms of the beginning and end points of the segments of the refractive index profile. FIG. 1 illustrates the definitions of radius used herein. The radius of the center index segment 10, is the length 2 that extends from the waveguide centerline to the point at which the profile becomes the .alpha.-profile of segment 12, that is, the point selected to start the calculation of the relative index using the .alpha.-profile equation. The radius of segment 12 extends from the centerline to the radial point at which the extrapolated descending portion of the .alpha.-profile crosses the extrapolated extension of profile segment 14. The radius of segment 14 extends from the centerline to the radius point at which the .DELTA. % is half the maximum value of the .DELTA. % of segment 16. The width of segment 16 is measured between the half .DELTA. % percent values of segment 16. The radius of segment 16 extends from the centerline to the midpoint of the segment. PA1 The effective area is EQU A.sub.eff =2.pi.(.intg.E.sup.2 r dr).sup.2 /(.intg.E.sup.4 r dr), PA1 The profile volume is defined as 2.intg..sub.r1.sup.r2 .DELTA. % r dr. The inner profile volume extends from the waveguide centerline, r=0, to the crossover radius. The outer profile volume extends from the cross over radius to the last point of the core. The units of the profile volume are % .mu.m.sup.2 because relative index is dimensionless. The profile volume units, % .mu.m.sup.2, will be referred to simply as units throughout this document. PA1 The crossover radius is found from the dependence of power distribution in the signal as signal wavelength changes. Over the inner volume, signal power decreases as wavelength increases. Over the outer volume, signal power increases as wavelength increases. PA1 The initials WDM represent wavelength division multiplexing. PA1 The initials SPM represent self phase modulation, a non-linear optical phenomenon wherein a signal having a power density above a specific power level will travel at a different speed in the waveguide relative to a signal below that power density. SPM causes signal dispersion comparable to that of linear dispersion having a negative sign. PA1 The initials FWM represent four wave mixing, the phenomenon wherein two or more signals in a waveguide interfere to produce signals of different frequencies. PA1 The term, .DELTA. %, represents a relative measure of refractive index defined by the equation, EQU .DELTA.%=100.times.(n.sub.i.sup.2 -n.sub.c.sup.2)/2n.sub.i.sup.2, PA1 The term alpha profile, .alpha.-profile refers to a refractive index profile, expressed in terms of .DELTA.(b) %, where b is radius, which follows the equation, EQU .DELTA.(b)%=.DELTA.(b.sub.o)(1-[.vertline.b-b.sub.o.vertline./(b.sub.1 -b.sub.o)].sup..alpha.), PA1 The pin array bend test is used to compare relative resistance of waveguide fibers to bending. To perform this test, attenuation loss is measured when the waveguide fiber is arranged such that no induced bending loss occurs. This waveguide fiber is then woven about the pin array and attenuation again measured. The loss induced by bending is the difference between the two attenuation measurements. The pin array is a set of ten cylindrical pins arranged in a single row and held in a fixed vertical position on a flat surface. The pin spacing is 5 mm, center to center. The pin diameter is 0.67 mm. The waveguide fiber is caused to pass on opposite sides of adjacent pins. During testing, the waveguide fiber is placed under a tension just sufficient to make the waveguide conform to a portion of the periphery of the pins.
The concept of adding structure to the waveguide fiber core by means of core segments, having distinct profiles to provide flexibility in waveguide fiber design, is described fully in U.S. Pat. No. 4,715,679, Bhagavatula. The segmented core concept can be used to achieve unusual combinations of waveguide fiber properties, such as those described herein.